Just to clarify my post #2. I didn't intend to imply that the Mexicans only turned UK crews over, they appear to do the same to all aircrew. The checks, as noted above, are perfunctory and clearly intended to make some sort of political point,I suspect, at the way Mexican crews (and maybe Mexicans nationals in general) are treated by various immigration and customs worldwide.
I do have some sympathy for the comments about coming in via Heathrow, but to be fair UK crews get a pretty intensive scrutiny (sometimes overly intrusive) going through their own security. Whereas, I find USA TSA checks are designed to be sensible and expeditious at all the airports from a security viewpoint, but the Immigration guys are purely there to make your life a misery! No-ones perfect, apart from the Dominican Republic, where you just wave your airline pass and walk through! Oh for the old days.